“Mirth is like a flash of lightning, that breaks through a gloom of clouds, and glitters for a moment; cheerfulness keeps up a kind of daylight in the mind, and fills it with a steady and perpetual serenity.” –Joseph Addison
One of the most brilliant phenomena found in our atmosphere is that of a lightning strike.
Shown below in breathtaking slow-motion, somewhere on the order of 100,000,000,000,000,000,000 electrons are exchanged in a single bolt between the clouds and the Earth’s surface.
How does this happen?
Remember that every atom in the Universe — including in our atmosphere — is made up of a positively charged nucleus and a whole slew of negatively charged electrons. While we normally think of atoms as neutral, with the number of electrons equal to the number of protons in each atom’s nucleus, this is not always the case.
Because, quite often, it’s energetically favorable for an atom to become ionized, where it either picks up or loses an electron (or two, or three). Something as simple as table salt is an example of a few ions.
Now, if you can separate these ions from one another, you create a separation of charge, which creates a Voltage. When the Voltage (also known as the electric potential difference) between two regions becomes too great — even if air is the only thing between them — it will spontaneously become conductive, and the rapid exchange of charge is what you see as a lightning strike!
You’re familiar with lightning happening over great distances, as charge gets transferred from clouds in a thunderstorm down to the Earth itself. But, as I showed you last year, when Iceland’s Eyjafjallajökull erupted, oftentimes volcanic eruptions produce lightning as well!
There are some amazing pictures of volcanic lightning that have been shot over the years. Perhaps my favorite is of last year’s Eyjafjallajökull eruption, as photographed by helicopter!
While it’s been notoriously difficult to catch volcanic lightning in action historically, the feat has been accomplished many times, for many different volcanoes.
For example, here’s Chile’s Chaitin volcano, from its recent 2008 eruption, its first in 9000 years!
Japan’s Sakurajima, an incredibly active volcano in recent history, has been erupting almost continuously since 1955. A Volcano Observatory was set up to continuously monitor its activity in 1960, and has observed volcanic lightning many times, including in this 1988 outburst.
In fact, volcanic lightning has been captured on film as far back as the 1944 eruption on Mount Vesuvius!
Now, I’d love to tell you exactly how volcanic lightning works, but to be completely honest, much as is the case for normal, thunderstorm-based lightning, we aren’t 100% sure; it’s still an area of active research. (See here and here for some examples.)
But, as a theoretical physicist, I can certainly give you a general picture for what is very likely going on here to cause this, and expand upon what I’ve previously told you.
Atoms, for the most part, start out neutral. (That’s step 1.) But with lots of free energy present, it’s certainly no problem to knock the electrons off of some atoms that hold onto them loosely, while at the same time atoms that are keen to pick up these newly freed electrons can do just that. (Step 2.)
That part is absolutely no problem: remember, it’s a volcano!
With temperatures of around 1500 Kelvin, there’s certainly enough energy floating around to knock electrons off of some of the atoms that hold onto them most loosely, where they can subsequently be picked up easily by other atoms, creating a large number of both positive and negative ions.
Now, the key thing that needs to happen — from this point — is we’ve got to separate the negative charges from the positive ones. (That’s step 3.) And we have to separate enough of them, over an interesting enough distance, to get an electric potential difference that will cause a lightning strike! (Step 4.) If we can do that, we can make volcanic lightning.
So how could we separate these charges? Remember what we’ve got here: a bunch of ionized atoms — both positive and negative ions — in a hot, turbulent environment. Coming up from the depths of the Earth, we should have a great many elements of interest involved here.
Right away, one of the first things you notice about these elements is that they have different masses from one another, as well as different radii! Now, they should all come out at a high temperature, which cools over time, once they leave the volcano itself. This is very important for the speeds of the atoms/ions in question.
In general, when the atoms and ions first come out, they’re moving more quickly, and they cool over time, slowing down.
That’s no big deal on its own, but there are two other very important factors that make it very easy to separate these positive and negative charges from one another. First off, these ions have very different masses from one another!
The heavier the atomic weight of an element is, the slower it moves, even at the same temperature as a lighter element! That means a lot of different things, including that heavier ions have more inertia, and it’s more difficult to change their momenta. So these slow-moving, heavy ions will move around very differently to fast-moving, light ones. Moreover, this is true at all temperatures!
The second very important factor that makes it easy to separate these ion-types from one another? The tremendous difference in sizes — and hence cross-sections — between positive and negative ions.
Sure, elements have different sizes from one another in the fashion I showed you above. But ions work in a much more dramatic fashion! Let’s take a look at exactly how.
In general, negative ions are huge, and positive ions are tiny! Why is that? You put more electrons on your atom, and they repel one another; the nucleus (with fewer protons than there are electrons in this ion) cannot hold the electrons as tightly as it could if the atom were neutral, and the atom increases in size. On the other hand, to become a positive ion, you kick electrons out of your atom, and the nucleus (with more protons that there are electrons in the ion) holds onto the electrons more tightly than before!
This means that the negative ions have larger cross-sections than the positive ions, and hence they interact very differently than the positive ions do!
Combine these things together: different mass ions moving at different average speeds with different cross sections in an environment with a temperature gradient, and there’s your charge separation! And what does that give you?
Volcanic lightning! The above, and all the subsequent photos, are of this June’s Chile ash cloud, seen both over Chile and Argentina. These are some of the most recent — and most spectacular — photos of volcanic lightning ever taken. Enjoy!
And as an added bonus, those of you on google+ can check out an entire album of volcano/volcanic lightning photos here!